Electric motors, or devices for transducing electrical power into mechanical power (or vice versa), have been of commercial interest for many years due to their wide variety of applications ranging from operation of industrial systems to personal appliances. The term “electric motor” as used herein refers to electric motors and electric generators.
The continuous operation torque density of an electric motor for a given type, dimension, and volume, is primarily limited by the ability of the motor to remove heat. The majority of the heat in an electric motor is produced by electrical resistance in the motor windings. Heat in a stator wrapped electric motor is conventionally transferred from the coil windings, through multiple layers of electrical insulation to the stator teeth, through the stator teeth to the stator back-iron, and through the stator back-iron to the stator housing where it may then diffuse into the environment. Poor thermal communication from the coil windings to the environment results in a buildup of heat in the stator slot limiting the amount of torque an electric motor may continuously transduce without causing permanent damage to its components.
High performance electric motors for mobile traction applications, where weight and volume are critical to total system performance, may utilize direct water cooling to increase cooling rates. While this may result in moderate increases in the continuous torque density for a given motor dimension, the tight packing increases total copper weight (a large cost driver for motors) and often requires custom laser welding assembly machines to automate production. In addition, water cooling requires the use of water tight bearings and seals, which may also be a source of system failure.
Thermal buildup in electric motors is exacerbated under high load low RPM applications where a combination of high electrical current, due to low back EMF, and low rotor fan speed often results in coil insulation and motor failure. Unfortunately, many commercial, industrial, and traction applications require high shaft torque at low RPMs.
Heat buildup in an electrical circuit is a function of i2R; therefore, a moderate increase in the electrical current for a given wiring (electrical resistance, R) will result in a significant increase in heat production. I2R losses in a typical industrial electrical motor account for 40-65% of all motor losses, far higher than any other single source of loss. The electrical current necessary to generate a given field is determined by the permeability of the magnetic circuit. The biggest decrease in magnetic permeability (increase in magnetic reluctance) is the airgap separating the stator and the rotor. Airgaps, typically in the range of 0.004 to 0.010 inches may decrease the relative magnetic permeability of a circuit by over 100 times, therefore requiring 100 times more magnetizing current (in amp-turns) to generate the same field strength. While airgaps are engineered to be as small as possible, machining tolerances limit the minimum tolerance necessary to prevent the collision between the stator and rotor, often referred to as “walling,” which will rapidly damage the motor. Conventional motor design has worked for years to optimize the material selection and airgap length based on commercially reasonable manufacturing techniques. Once the reluctance of the magnetic circuit has been optimized then the only source of further performance improvement is decreasing electrical resistance.
Conventional methods of wiring stators using welded precision-formed rectangular wires have recently been developed to increase the ratio of stator slot volume filled by coil windings. These methods have resulted in volumetric slot fill ratios of up to 73%, an increase of 82% over conventional stators with round wire. The larger wire cross-sectional area results in lower electrical resistance and increased thermal conductivity to the stator. Unfortunately, the increased slot fill also prevents fluid, such as water or air, from directly cooling the coil windings, requiring the majority of the heat to be first transferred to the stator before it may diffuse to the environment. The tight packing also increases total copper weight (a large cost driver for motors) and often requires custom laser welding assembly machines to automate production.
Even though electric motors, such as single phase and three phase induction motors, have been commercially produced to exceed 90% thermodynamic efficiency during normal operation, their relatively low torque density for a given power and weight often requires the use of mechanical transmissions, such as gearboxes or belt drives, to increase torque applied to an output shaft. Unfortunately, typical transmission efficiencies range from 40% or less for worm gears, to 90% for single stage properly sized belt or planetary gear transmissions; thereby, decreasing the total electric-gearbox system efficiency to less than 70% during normal use in most applications.
To further complicate matters, higher RPM motors require higher electrical switching rates, which may result in increased losses due to eddy currents formed in the stator and rotor laminations. While thinner laminations may be used to decrease eddy currents, this often adds to the cost of manufacturing and may result in decreased overall field density.
Therefore, in order to further increase the continuous operation torque and power density of electric motors there is a need for new designs that enable more efficient transduction of electrical energy into mechanical energy.